One of the most exciting potential new memory technologies is magnetic random access memory (MRAM) based on advanced spintronics, which promises to be a high performance, non-volatile memory. The essential feature of MRAM is the switching of a magnetic tunnel junction (MTJ) memory cell between two distinct resistance states associated with the relative magnetic orientation of the ferromagnetic electrodes sandwiching the tunnel barrier. This switching can be achieved by passing spin polarized charge currents directly through the MTJ, so that switching is induced via spin transfer torque (STT); however, the current densities that are required are currently too large to make it a viable technology. A number of different approaches are being pursued to decrease the current density, using novel materials and physics. In particular, there has been a great deal of interest in the generation of spin currents without any significant charge currents through the use of temperature gradients (i.e., ‘spin-caloritronics’). However, the demonstration of the potential of thermally induced spin currents for MRAM has not been realized due to the difficulty in creating sufficiently large temperature gradients across the ultra-thin tunnel barriers needed for useful applications.
On the other hand, using heat to create gradients and charge-currents has been an active area of research in thermoelectrics (1). Spin caloritronics (2, 3) adds a new dimension to this concept by employing heat to create spin-dependent chemical potential gradients in ferromagnetic materials (4). Traditionally, electric current driven spin-currents have been used to transport spin angular momentum to change the magnetization of a magnetic material a phenomenon known as spin-transfer-torque (STT) (5-7). Heat currents can also create spin-currents in magnetic materials; the transfer of spin angular momentum through this process has been called thermal-spin-torque (TST) (8, 9). A number of experiments employing spin currents generated by heat have been reported, including: the spin-Seebeck effect observed in ferromagnetic metals (10, 11), semiconductors (12) and insulators (13); thermal spin injection from a ferromagnet into a non-magnetic metal (14); the magneto-Seebeck effect observed in magnetic tunnel junctions (15-17); Seebeck spin tunneling in ferromagnet-oxide-silicon tunnel junctions (18); and several others (19, 20). On the other hand, while there have been several theoretical predictions (8, 9, 21, 22) of the TST, few experiments have been reported to date.